Re: Unicode confusables and reordering characters considered harmful

[Gregory Heytings <gregory@heytings.org>]

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>
> FWIW, I think the source would be clearer if it used `?\u{CHARNAME}` 
> instead of the literal chars.
>

I did not know that feature exists, thank you!

Here's the updated patch, which is indeed much more readable.

And a screenshot of TUTORIAL.he, which doesn't seem unreadable at all (but 
of course that feature would need to be turned off for beginners who read 
TUTORIAL.he, it's meant for source code, not for prose).

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From f9354f99d1d27212659cbee0420b35e883373a5d Mon Sep 17 00:00:00 2001
From: Gregory Heytings <gregory@heytings.org>
Date: Wed, 3 Nov 2021 19:22:07 +0000
Subject: [PATCH] Make bidi reordering characters visible

* lisp/progmodes/prog-mode.el (bidi-reordering-characters-visible,
bidi-reordering-characters-fontify,
bidi-reordering-characters-toggle-visibility): New functions.
---
 lisp/progmodes/prog-mode.el | 41 ++++++++++++++++++++++++++++++++++++-
 1 file changed, 40 insertions(+), 1 deletion(-)

diff --git a/lisp/progmodes/prog-mode.el b/lisp/progmodes/prog-mode.el
index db350a5f70..7fcdc7b5e3 100644
--- a/lisp/progmodes/prog-mode.el
+++ b/lisp/progmodes/prog-mode.el
@@ -293,6 +293,44 @@ turn-on-prettify-symbols-mode
 (define-globalized-minor-mode global-prettify-symbols-mode
   prettify-symbols-mode turn-on-prettify-symbols-mode)
 
+(defvar-local bidi-reordering-characters-visible nil
+  "Internal variable used by `bidi-reordering-characters-visible'.")
+
+(defun bidi-reordering-characters-fontify ()
+  "Fontify bidi reordering characters with `font-lock-warning-face'."
+  (font-lock-add-keywords
+   nil
+   '(("\N{LEFT-TO-RIGHT EMBEDDING}\\|\N{RIGHT-TO-LEFT EMBEDDING}\\|\
+\N{LEFT-TO-RIGHT OVERRIDE}\\|\N{RIGHT-TO-LEFT OVERRIDE}\\|\
+\N{LEFT-TO-RIGHT ISOLATE}\\|\N{RIGHT-TO-LEFT ISOLATE}\\|\
+\N{FIRST STRONG ISOLATE}\\|\N{POP DIRECTIONAL FORMATTING}\\|\
+\N{POP DIRECTIONAL ISOLATE}" . (0 'font-lock-warning-face t)))))
+
+(defun bidi-reordering-characters-visible ()
+  "Display bidi reordering characters as arrows."
+  (setq buffer-display-table (or buffer-display-table
+                                 standard-display-table
+                                 (make-display-table)))
+  (bidi-reordering-character-toggle-visibility)
+  (add-hook 'font-lock-mode-hook #'bidi-reordering-characters-fontify))
+
+;;;###autoload
+(defun bidi-reordering-character-toggle-visibility ()
+  "Toggle the visibility of bidi reordering characters."
+  (interactive)
+  (setq bidi-reordering-characters-visible
+        (not bidi-reordering-characters-visible))
+  (let ((v bidi-reordering-characters-visible))
+    (aset buffer-display-table ?\N{LEFT-TO-RIGHT EMBEDDING} (if v [?→] nil))
+    (aset buffer-display-table ?\N{RIGHT-TO-LEFT EMBEDDING} (if v [?←] nil))
+    (aset buffer-display-table ?\N{LEFT-TO-RIGHT OVERRIDE} (if v [?→] nil))
+    (aset buffer-display-table ?\N{RIGHT-TO-LEFT OVERRIDE} (if v [?←] nil))
+    (aset buffer-display-table ?\N{LEFT-TO-RIGHT ISOLATE} (if v [?→] nil))
+    (aset buffer-display-table ?\N{RIGHT-TO-LEFT ISOLATE} (if v [?←] nil))
+    (aset buffer-display-table ?\N{FIRST STRONG ISOLATE} (if v [?↓] nil))
+    (aset buffer-display-table ?\N{POP DIRECTIONAL FORMATTING} (if v [?↑] nil))
+    (aset buffer-display-table ?\N{POP DIRECTIONAL ISOLATE} (if v [?↑] nil))))
+
 ;;;###autoload
 (define-derived-mode prog-mode fundamental-mode "Prog"
   "Major mode for editing programming language source code."
@@ -300,7 +338,8 @@ prog-mode
   (setq-local parse-sexp-ignore-comments t)
   (add-hook 'context-menu-functions 'prog-context-menu 10 t)
   ;; Any programming language is always written left to right.
-  (setq bidi-paragraph-direction 'left-to-right))
+  (setq bidi-paragraph-direction 'left-to-right)
+  (bidi-reordering-characters-visible))
 
 (provide 'prog-mode)
 
-- 
2.33.0


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